In the past, highly saturated nitrile rubber obtained by hydrogenating the butadiene part of nitrile rubber (acrylonitrile-butadiene copolymer rubber) has been known to exhibit excellent resistance to liquefied gas such as fluorohydrocarbon and carbon dioxide (liquefied gas resistance) and has been utilized as a fluorohydrocarbon seal member, carbon dioxide seal member, etc. of a refrigerant of refrigerating machines and air-conditioners.
On the other hand, an air-conditioning use refrigerating machine sometimes has a maximum temperature reaching as much as 140 to 150° C., so the rubber material used for it is required to have a 150° C. or more heat resistance. Further, as one important physical property of a seal member for a refrigerating machine, it is required that even if rendered a high temperature state after being made to contact a refrigerant of fluorohydrocarbon or carbon dioxide (seal member contacts fluorohydrocarbon or carbon dioxide in state of liquefied gas), the rubber not blister or crack.
In view of such a situation, for example, Patent Document 1 discloses a rubber composition for forming a seal member used for sealing a fluid at a pressure of 0.1 MPa to 15 MPa and containing hydrogenated carboxylated nitrile rubber. However, the cross-linked rubber obtained using the rubber composition described in Patent Document 1 was not sufficient in compression set resistance. In particular, when used for a seal member application for a refrigerating machine, it is necessary to hold a high pressure and further to repeatedly raise and lower the pressure, so even more than the past, excellent mechanical strength such as tensile strength and elongation and low compression set are being demanded. As opposed to this, the art of Patent Document 1 was not able to sufficiently meet this demand. In addition, the cross-linked rubber obtained using the rubber composition described in Patent Document 1 was excessively high in hardness and therefore the assembly ability and sealability when used as a seal member were inferior.